The measurement of the substrate temperature is one of the most important aids for the control of coating processes. The crystallization behaviour, the rate of epitaxial growth and diffusion, etc., are all functions of the substrate temperature which influences the thermodynamic, chemical, and physical processes. The measurement of temperature is of significant importance in the fields of the manufacture of: semiconductor and electronic components, coated optics, high-capacity optical components, integrad optical circuits ("IOC"), semiconductor laser diodes, etc.
This is also true for all processes where coating technologies are used, e.g. chemical vapor deposition ("CVD"), molecular beam epitaxy ("MBE"), thermal oxidation, and cathode sputtering or plasma polymerization.
As a consequence of various process conditions--e.g., high temperatures, ultra high vacuum ("UHV"), chemically reactive surroundings, rotating substrates)--direct measurement of the substrate temperature via calibrated platinum film resistances (i.e., thermocouples) or other contact thermometers is typically not possible. Accordingly, the substrate temperature is usually determined by means of pyrometrical measurements. Because of interference at the interfaces of the growing layers, the measured temperature-radiation intensity is subject to changes in the temperature T as well as to variations of layer/coating-thickness d (film).
Therefore, during the coating process, the pyrometer signal will oscillate due to the changing layer-thickness even if the temperature remains constant.
As a consequence of this interference phenomenon, the emissivity .epsilon. continuously changes during coating so that a pyrometrical measurement of temperature is not feasible. Particularly problematic is the pyrometrical measurement of temperature in multiple layer systems where the actual emissivity depends on: the thickness of all the layers, the optical constants of all the layers, the temperature dependencies of the optical constants, the angle of incidence, and the monitoring wavelength.
In situ measurement systems for the real time determination of temperature during coating have been known only since 1988. The procedure described in E. S. Hellmann and J. S. Harris, J. Crys. Grow., 81, 38-42, (1988) is based on the temperature dependence of the bandgap of semiconductor wafers and can only be used in processes where the process chamber geometry allows for a transmission measurement and where the band edge of the substrate is in the detectible spectroscopic field. Thus, for example, the temperature of quartz or metal substrates cannot be measured.
Therefore, the procedure has been restricted to molecular beam epitaxy ("MBE") arrangements designed for indium free mounting, i.e. the substrate is not, as usual, fixed on a molybdenum block, but rather is mounted directly in front of the heating elements.
Furthermore, in situ temperature measurement methods depend on the functional relationship between the refractive-index and temperature (e.g., ellipsometrical temperature measurement), which requires an exact knowledge of the temperature dependence of the material constants. Since this functional relationship is not known for most materials, these methods are rarely used.
Therefore, with the current state of the art an in situ measurement of temperature of the substrate during coating is either not possible in principle or not practicable.